SeaMoby Working Group L-N. Hamer Internet Draft B. Kosinski Document: draft-hk-seamoby-ct-ipsec-00.txt Nortel Networks May 28, 2001 IPSec Context Transfer Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet- Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html The distribution of this memo is unlimited. This memo is filed as <draft-hk-seamoby-ct-ipsec-00.txt>, and expires November 2001. Please send comments to the authors. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. 1. Abstract There are a large number of IP access networks where one may wish to provide security for end user traffic, or secure the access network from unauthorized traffic. One protocol which may be used to provide these services is IPSec, which requires a node to establish a security association with the access network in order to obtain these services. Traditionally, such an association is considered static, however, there are many situations in which the ability to move an IPSec security association (SA) from one security gateway (SG) to another within the access network may be beneficial. Examples of this include IPSec handover in mobile LANs and PANs, load-balancing between IPSec SGs, and fail-over applications where high-availability is required. Currently, in order to perform this Hamer,Kosinski Expires November 2001 [Page 1]
Internet Draft IPSec context transfer transfer, it would be necessary to terminate the existing SA and re- negotiate a new SA at the new SG. However, this approach may be inappropriate in cases where high performance is required. Thus, in such cases, the ability to directly transfer an SA from one SG to another would be useful. The intent of this draft is to describe the unique requirements for transfer of IPSec context and to detail the specific data which must be transferred in order to move an IPSec SA. In addition, a number of unique issues regarding IPSec context transfer will be addressed, and some potential solutions discussed. 2. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC-2119]. Hamer,Kosinski Expires November 2001 [Page 2]
Internet Draft IPSec context transfer 3. Introduction There are a large number of IP access networks where one may wish to provide security for end user traffic, or secure the access network from unauthorized traffic. One protocol which may be used to provide these services is IPSec, which requires a node to establish a security association with the access network in order to obtain these services. Traditionally, such an association is considered static, however, there are many situations in which the ability to move an IPSec security association (SA) from one security gateway (SG) to another within the access network may be beneficial. Examples of this include IPSec handover in mobile LANs and PANs, load-balancing between IPSec SGs, and fail-over applications where high-availability is required. Currently, in order to perform this transfer, it would be necessary to terminate the existing SA and re- negotiate a new SA at the new SG. However, this approach may be inappropriate in cases where high performance is required. Thus, in such cases, the ability to directly transfer an SA from one SG to another would be useful. Much of the work in establishing a generic context transfer framework has already begun [2][3]. These documents focus on the generic requirements and framework for context transfer. However, one must identify, for each feature to which context transfer is applicable, the data which must be transferred, and any unique requirements which are relevant. This document attempts to define the contents of the IPSec feature context and the specific requirements applicable to IPSec. In addition, a number of unique issues regarding the transfer of IPSec context will be discussed, along with potential solutions. 4. Terminology Much of the terminology used in this document is the same as that found in [2]. However, a number of additional definitions are provided below: o SA - Security association. Defined in [4] as a "simplex æconnectionÆ that affords security services to the traffic carried by it." o SG - Security gateway. A network entity with which a node may establish one or more security associations, either from the node to the gateway or vice versa. Hamer,Kosinski Expires November 2001 [Page 3]
Internet Draft IPSec context transfer 5. Reasons for IPSec CT Currently, the only approaches for re-establishing an IPSec session involve tearing down the old IPSec SAs and establishing new SAs with the aid of a key exchange protocol such as IKE [7] or KINK. Unfortunately there are many problems with this approach, the main ones being long latency and excessive signalling during handover. This document addresses another approach, where access routers exchange state information directly. This approach has many advantages, such as reduced latency during handover and minimal signalling from the mobile node. 6. Network Model When discussing IPSec context transfer between SGs, the following network model will be assumed: +- Access Network -+ +--------+ | +-----+ | +----------+ +------+ | Mobile |===|=====| SG1 |------|--|----------|---| End | | Node | | +-----+ | | | | | +--------+ | | | Internet | | Host | | ... | | | +------+ | +-----+ | +----------+ | | SGm | | | +-----+ | +------------------+ Where context transfer may occur between any two SGs in the access network. It is assumed that the SA between the MN and SG is an IPSec SA in tunnel mode. Either AH [5] or ESP [6] may be used depending upon whether or not encryption is required. 7. Context Transfer Requirements All of the specific requirements defined in [3] are applicable to the transfer of IPSec context. In addition, one of the primary requirements of IPSec is that the identity of the security gateway MUST be known to the mobile node, in order to properly encapsulate packets for transmission. 7.1 Discovery and Update of SG Identity at MN Due to the peer to peer nature of the IPSec architecture, it is necessary for the nodes participating in an SA to know the identity of each other. Under normal circumstances, this is not an issue. However, in the case of context transfer, the identity of the SG may change on-the-fly. As a result, there must be some way to ensure Hamer,Kosinski Expires November 2001 [Page 4]
Internet Draft IPSec context transfer that the mobile node transmits packets to the correct SG, even after a context transfer. There are two primary solutions to this issue: Direct SG Communication: Direct SG communication requires that the node be able to discover the address of the initial SG through some means (DHCP, DNS, etc). In addition, during handover to a new SG, the node must be notified of the new SG address through some form of signaling so that the local SAD in the MN may be updated to reflect the address of the new SG. Indirect SG Communication: This form of communication requires some form of tunnel to be set up from the node to a virtual SG. This is achieved with the use of a virtual address for the SG. The node must somehow retrieve (via DHCP, DNS, etc) the virtual address necessary to communicate to the SG currently serving the MN. Then, during handover, the network takes care of correctly re-directing traffic destined to the new SG, making the process transparent to the mobile node. Each method has its pros and cons. Direct communication reduces the complexity in the network but requires additional signaling, and thus added latency. The indirect form requires added complexity at the network, but is transparent to the node. One must weigh these factors when considering an appropriate mechanism for solving this problem. 7.2 PMTU Rediscovery The IPSec architecture, as defined in [4], requires that the nodes participating in an SA be aware of the Path MTU over which the treated packets are traveling. However, if a context transfer occurs, and the new SG is in another location in the network, it is possible for the PMTU of the underlying network to change. This is not a problem if the PMTU of the new path is greater than that of the old path. However, if the PMTU decreases, this may cause problems for applications which rely on knowledge of the PMTU. Possible solutions to this problem may include PMTU rediscovery, or even network engineering to avoid the problem entirely. However, this issue is really one of general context transfer, and thus will not be discussed here. 7.3 SA Conflict Resolution During a context transfer, it is possible that the new SG which has been targeted as the candidate for context transfer may not be able to support the SAs being transferred (i.e., unavailability of ciphering algorithms, etc). The method for dealing with this Hamer,Kosinski Expires November 2001 [Page 5]
Internet Draft IPSec context transfer situation is beyond the scope of this document, but may include selection of a new candidate SG, or the termination of the IPSec SAs, forcing the mobile node to establish a new SA pair with the new SG, allowing for re-negotiation of the SA parameters. 8. IPSec Feature Context When determining the contents of the IPSec feature context, one must examine all the state, which is maintained at the SG. The actual data, which is stored in the gateway is collected in the Security Policy Database (SPD) and the Security Association Database (SAD) [5]. The security policy database contains some static entries, containing general policies, which are established by the operator of the access network, and should not be transferred. Generally, these SPD entries are the same on all SGs within the operator domain. The SPD also contains selector parameters used to support SA management to facilitate control of SA granularity. In fact, an SA may be fine-grained or coarse-grained, depending on the selectors used to define the set of traffic for the SA. The selectors used to define the SA MUST be context transferred. Selector Fields: Source and Destination IP Address Source and Destination Port Transport Layer Protocol Name Sensitivity Level These fields are used by the gateway to identify packets for inbound and outbound processing. Note, fields not used to match packets against this SA MAY be omitted. Therefore, if, for example, only the source and destination IP addresses are used as a selector, the other fields MAY be excluded. For inbound processing, the following packet fields are used to look up the SA in the SAD: Outer HeaderÆs Destination IP address. IPSec Protocol (AH or ESP) SPI: the 32-bit value used to distinguish among different SAs terminating at the same destination and using the same IPSec protocol. These fields are used by a gateway to look up the SA in the SAD. Therefore, they MUST be context transferred. Hamer,Kosinski Expires November 2001 [Page 6]
Internet Draft IPSec context transfer Treatment Fields: Sequence Number Sequence Number Overflow Flag Antireplay Window AH Algorithm, keys, etc ESP Encryption Algorithm, keys, IV Mode, IV, etc ESP Authentication Algorithm, keys, etc Lifetime Protocol Mode Path MTU Treatment fields are used by the IPSec stack in actually processing the packets, once it has been determined that they must be treated. Again, fields which are not applicable to this SA MAY be omitted. For example, depending on the protocol mode, either the ESP or AH fields need not be transferred. Others may not need to be transferred depending on the IPSec implementation (for example, some IPSec stacks allow disabling of sequence number checking, thus these fields may not need to be transferred). Note, there are a number of fields in the IPSec context which are used to identify replay attacks in real time. These include the anti-replay window and the sequence number. These fields may initially cause concern, as they must be updated in real time, and should reflect the current state of the IPSec SA. The concern is that these fields may not be entirely accurate after context transfer because of the loss of some user packets. Careful considerations reveal this is not a problem. In fact, IPSec anti- replay functionalities were designed to accommodate minor packet loss [4]. The only major problem that may occur during the context transfer of an IPSec SA is when the SPI value of an IPSec SA to be transferred is already in use at the new SG. In this scenario, three possible solutions are to be considered: -Deny the context transfer. -Accept the context transfer but force re-negotiation of the IPSec SA. -Assign a new SPI entry unused at the new SG and signal this information back to the mobile node. This operation MUST be secure since it may open up some security holes. 9. Security Considerations Careful consideration needs to be taken to ensure that the context transfer of an IPSec SA is secure, especially when transferring SA information such as keys. In fact, as defined in [3], context transfer MUST be secure. How to secure the context transfer is Hamer,Kosinski Expires November 2001 [Page 7]
Internet Draft IPSec context transfer dependent on the network environment. In a trusted environment, no additional security mechanism is needed. But in an un-trusted environment, a security mechanism MUST be utilized. In order to keep the context transfer protocol simple, re-use of existing security technologies is recommended. All security requirements MAY be provided at the network layer with IPSec or at the transport layer with TLS. 10. References [1] S. Bradner, "keywords for use in RFCs to Indicate Requirement Levels", RFC2119 (BCP), IETF, March 1997. [2] The seamoby CT design team, "Context transfer: problem statement", draft-ietf-seamoby-context-transfer-problem-stat- 00.txt. [3] The seamoby CT design team, "General Requirements for a Context Transfer Framework", draft-ietf-seamoby-ct-reqs-00.txt. [4] S. Kent et. Al., "Security Architecture for the Internet Protocol" RFC-2401, November 1998 [5] Kent, S., and R. Atkinson, "IP Authentication Header", RFC 2402, November 1998. [6] Kent, S., and R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. [7] Harkins, D., and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. 11. Acknowledgments The authors would like to thank to following people for their useful comments and suggestions related to this draft: Hamid Syed, Gary Kenward, Jerry Chow and Bill Gage. 12. Author's Addresses Louis-Nicolas Hamer Nortel Networks Ottawa, ON CANADA Email: nhamer@nortelnetworks.com Hamer,Kosinski Expires November 2001 [Page 8]
Internet Draft IPSec context transfer Brett Kosinski Nortel Networks Ottawa, ON CANADA Email: brettk@nortelnetworks.com Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organisations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into. Expiration Date This memo is filed as <draft-hk-seamoby-ct-ipsec-00.txt>, and expires November 2001. Hamer,Kosinski Expires November 2001 [Page 9]